1. Field of the Invention
The present invention pertains generally to steam desuperheaters or attemperators and, more particularly, to a uniquely configured spray nozzle assembly for a steam desuperheating or attemperator device is, the spray nozzle assembly being adapted to improve the atomization performance of the nozzle at very low flow rates. In one embodiment, the spray nozzle sub-assembly of the spray nozzle assembly comprises a fixed nozzle element which is integrated into a spring-loaded nozzle element. The spray nozzle sub-assembly is specifically adapted to improve water droplet fractionation at lower flow rates through the use of only the smaller, central fixed nozzle element, and at high flow rates through the concurrent use of the fixed and spring-loaded nozzle elements. Though at low flow rates, the spring-loaded nozzle element is generally ineffective in water fractionation, high flow rates facilitate the transmission of two spray cones from spray nozzle sub-assembly, one associated with the fixed nozzle element being positioned within one associated with the spring-loaded nozzle element. The double spray cone is able to provide good results at high flow rates by producing an effectively higher spray area through the formation of two water cones (rather than a single water cone), such water cones being sprayed into a flow of superheated steam in order to reduce the temperature of the steam. In another embodiment, the spray nozzle sub-assembly of the spray nozzle assembly comprises a nested pair of spring-loaded primary and secondary nozzle elements which are also adapted to provide an effectively higher spray area through the formation of two water cones.
2. Description of the Related Art
Many industrial facilities operate with superheated steam that has a higher temperature than its saturation temperature at a given pressure. Because superheated steam can damage turbines or other downstream components, it is necessary to control the temperature of the steam. Desuperheating refers to the process of reducing the temperature of the superheated steam to a lower temperature, permitting operation of the system as intended, ensuring system protection, and correcting for unintentional deviations from a prescribed operating temperature set point. Along these lines, the precise control of final steam temperature is often critical for the safe and efficient operation of steam generation cycles.
A steam desuperheater or attemperator can lower the temperature of superheated steam by spraying cooling water into a flow of superheated steam that is passing through a steam pipe. Attemperators typically comprise one or more spray nozzles or nozzle assemblies positioned so as to spray cooling water into the steam flow. By way of example, attemperators are often utilized in heat recovery steam generators between the primary and secondary superheaters on the high pressure and the reheat lines. In some designs, attemperators are also added after the final stage of superheating. Once the cooling water is sprayed into the flow of superheated steam, the cooling water mixes with the superheated steam and evaporates, drawing thermal energy from the steam and lowering its temperature.
With regard to the functionality of any spray nozzle assembly of an attemperator, if the cooling water is sprayed into the superheated steam pipe as very fine water droplets or mist, then the mixing of the cooling water with the superheated steam is more uniform through the steam flow. On the other hand, if the cooling water is sprayed into the superheated steam pipe in a streaming pattern, then the evaporation of the cooling water is greatly diminished. In addition, a streaming spray of cooling water will typically pass through the superheated steam flow and impact the interior wall or liner of the steam pipe, resulting in water buildup which can cause erosion, thermal stresses, and/or stress corrosion cracking in the liner of the steam pipe that may lead to its structural failure. However, if the surface area of the cooling water spray that is exposed to the superheated steam is large, which is an intended consequence of very fine droplet size, the effectiveness of the evaporation is greatly increased. Further, the mixing of the cooling water with the superheated steam can be enhanced by spraying the cooling water into the steam pipe in a uniform geometrical flow pattern such that the effects of the cooling water are uniformly distributed throughout the steam flow. Conversely, a non-uniform spray pattern of cooling water will result in an uneven and poorly controlled temperature reduction throughout the flow of the superheated steam. Along these lines, the inability of the cooling water spray to efficiently evaporate in the superheated steam flow may also result in an accumulation of cooling water within the steam pipe. The accumulation of this cooling water, in addition to potentially causing the problems highlighted above, will eventually evaporate in a non-uniform heat exchange between the water and the superheated steam, resulting in a poorly controlled temperature reduction.
In the current generation of combined cycle power plants, there is an increased interest in reducing the minimum load to which the plant is able to operate. The manner of plant operation, often referred to as “park-load,” effectively reduces the minimum load of the plant as the power generated is produced with a bypass valve in a partial opening mode. This mode of operation requires that smaller flows of steam be quenched and controlled through the use of the aforementioned attemperators.
However, the designs of the spray nozzle assemblies of currently know attemperators are not particularly well suited for “park-load” plant operation. In this regard, in many current nozzle assembly designs, the valve or spray nozzle element thereof is energized by a spring and is set to a prescribed break-up pressure as is controlled by an upstream control valve. The pressure drop on the nozzle assembly when the nozzle element thereof is actuated to its open position facilitates the generation of a cone of water that is broken into multiple droplets which are mixed into the flow of high temperature steam. However, when using such nozzle assemblies to cool steam at lower flow rates, a low pressure similar to the nozzle assembly break-up pressure will typically result in the generation of a single jet of water, rather than a cone-shaped flow of water mist, thus not guaranteeing good control of steam attemperation.
The present invention addresses these and other deficiencies of currently known spray nozzle assemblies. In this regard, various novel features of the present invention will be discussed in more detail below.